core: allow devices to provide their own device specific kernel headers

Currently, we need to pollute the common bionic/libc/kernel/...
directory with device specific kernel headers. This has created a giant
dumping ground of headers from all the projects we have ever done.

Allow each device to provide their own headers and thus eventually allow
bionic/libc/kernel to become *just* the common linux kernel headers.

This change will automatically add device/oem/name/kernel-headers to
the include list and also allow the BoardConfig.mk to specify
additional directories with TARGET_BOARD_KERNEL_HEADERS variable.
The requirement is that the final dir name must be kernel-headers to
mark explicitly where kernel headers are kept.

Change-Id: I7d32a979eef1f464db06c1bc448a04ddc3575efd
Signed-off-by: Dima Zavin <dima@android.com>